From the viewpoints of diversification of food culture, shortening of cooking time and demand for convenience, it has become frequent that so-called retort pouch food obtained by introducing previously cooked food into a pouch, sealing the pouch and subjecting it to pressing/heating sterilization is purchased, the retort pouch food with the pouch is heated in hot water when necessary and the contents are taken out of the pouch and taken as a meal. Such retort pouch foods have begun to spread not only for usual home use but also for business use, and therefore, packaging materials capable of packaging a large quantity of foods at once have been desired.
Such retort pouch foods are subjected to room-temperature preservation over a long period of time or occasionally subjected to refrigeration/freezing preservation, and therefore, films that become their packaging materials are required to have high heat-seal strength and low-temperature impact resistance so that they should not be broken from the heat seal parts. Moreover, after packaging the retort pouch foods and sealing the packaging materials, the retort pouch foods are subjected to retort sterilization treatment in a high-temperature high-pressure pot at a temperature of about 100 to 140° C., and therefore, also from the viewpoint of quality control of foods, heat resistance of such a level as is capable of enduring the treatment at the heat seal parts and retention of heat-seal strength are required for the packaging materials.
In order to prevent decrease in heat-seal strength after heating/sterilization treatment such as retort treatment even if the degree of prevention is slight, there have been made a large number of proposals, such as a method of using a propylene/α-olefin block copolymer constituted of 95 to 70% by mass of polypropylene block units and 5 to 30% by mass of elastomer block units for a heat-sealing layer (e.g., patent document 1) and a propylene-based polymer composition obtained by adding a specific ethylene/α-olefin random copolymer component (C) to a propylene polymer component (A) and a propylene/α-olefin random copolymer component (B) (e.g., patent document 2).
Even if the above propylene-based polymer composition or the like is used, however, decrease in heat-seal strength after heating/sterilization cannot be inhibited in some cases.                Patent document 1: Japanese Patent Laid-Open Publication No. 2000-255012        Patent document 2: Japanese Patent Laid-Open Publication No. 2003-96251        